1. Field of the Invention
This invention relates to a pressure sensing apparatus for regulating the flow rate of a liquid, and more particularly to a pressure switch apparatus used to detect small variances in pressure of a flowing liquid.
2. Description of Related Art
A silicon substrate used in semiconductor fabrication usually has an uneven surface, the result of the number of dielectric layers formed over the substrate to cover some structure such as a contact window or a capacitor. Semiconductor fabrication therefore, needs a planarization process to obtain an even substrate surface. Otherwise, because of the differences in relative elevation, the interconnect metal lines can not easily be formed and the precision of transferring a desired pattern onto the substrate is compromised.
Currently local planarization on the substrate surface is accomplished using spin-on glass (SOG). The SOG process includes two steps: coating and curing. SOG uses a solution that includes a dielectric material in a solvent, for spin coating onto the substrate. The dielectric material in solution fills the concave structures on the substrate to obtain a local planarization. The coating is followed by curing, which dries the residual solution of SOG out of the substrate in a thermal process that bakes the substrate, solidifying the SOG solution into a crystal structure such as SiO.sub.2.
FIGS. 1A-1C are flow diagrams schematically illustrating a typical planarization process on a semiconductor substrate. Referring to FIG. 1A, three metal layers 10, 12, and 14 are formed on the substrate, the surface of which is represented by a base line in FIGS. 1A-1C. Two trenches 11 and 13 exist between the metal layers 10, 12, and 14. Referring to FIG. 1A and FIG. 1B, a silicon dioxide layer 15 is formed over the substrate partially filling trenches 11 and 13, which thus become two concave regions 16 and 17. Referring to FIG. 1B and FIG. 1C, a SOG layer 18 is formed over the silicon dioxide layer 15 completely filling concave regions 16 and 17. Following the curing to solidify the SOG, the planarization process is complete.
FIG. 2 is a plot schematically illustrating a conventional system for regulating the pressure of a flowing liquid. Referring to FIG. 2, a container 32 contains chemical solution 36 and is covered by a specialized cover 30. There are a first opening 29 and a second opening 39 on the cover 30. The first opening 29 is attached to a duct 35, onto which a pressure sensing apparatus 34 is connected. Helium gas can be pumped into the container 32 through the duct 35. A duct 27 connects the second opening 39 to a spray head 26. Therefore, the chemical solution 36 can be forced out of the container 32 through the duct 27 by pressurizing the container 32 with helium gas via the first opening 38. The chemical solution 36 follows duct 27, which includes a valve 28 to allow or inhibit the flow of the chemical solution 36. When the valve 28 is open, the chemical solution 36 flows to the spray head 26, and is sprayed on a semiconductor substrate 22, located on a rotating platform 24. To control the volume of the chemical solution 36, the pressure of the helium gas used to drive the chemical solution 36 should be kept relatively constant so that the pressure sensing apparatus can regulate the pressure. Unfortunately, in this conventional system, there is no method to detect damage to the ends of duct 27. Damage to duct 27 can induce problems in precisely regulating the flow rate of the chemical solution 36. In addition, the accumulation of helium gas can also over-pressurize the container 32. This over-pressurization usually causes the chemical solution 36 to be imprecisely supplied.